1. Field of the Invention
One disclosed aspect of the embodiments relates to methods for manufacturing solid-state image pickup devices. More particularly, one disclosed aspect of the embodiments relates to a technique for forming a light-shielding film in a solid-state image pickup device including pixels having a light-shielding film.
2. Description of the Related Art
Regarding active-pixel solid-state image pickup devices such as complementary metal oxide semiconductor (CMOS) image sensors, configurations have been proposed in which a global electronic shutter function is implemented in pixels and in which pixels for focus detection are additionally included.
The global electronic shutter function refers to a function of starting and terminating accumulation of photoelectric charges at the same timings in a plurality of pixels arranged in a matrix form. In order to perform the global electronic shutter function, each pixel of a solid-state image pickup device has a photoelectric conversion element and a charge holding part that holds charges generated through photoelectric conversion for a predetermined period. In the solid-state image pickup device having the global electronic shutter function, each charge holding part holds charges for a period from when accumulation of photoelectric charges is terminated to when the charges are read out. If charges generated in parts other than the photoelectric conversion element flow into the charge holding part during this period, the charges may become a noise signal, and consequently degrade the image quality. To cope with such an undesirable charge inflow, Japanese Patent Laid-Open No. 2007-115803 discloses a configuration in which a second gate electrode of polysilicon is formed so as to be stacked above a first gate electrode and cover the charge holding part in order to block incident light. The second gate electrode described in Japanese Patent Laid-Open No. 2007-115803 prevents light from coming into the charge holding part and consequently suppresses generation of charges in parts other than the photoelectric conversion element, thereby suppressing image quality degradation.
In addition, Japanese Patent Laid-Open No. 2009-105358 discloses a configuration including focus detection pixels, each of which is provided with a light-shielding film having a slit.
With the recent increases in the number of pixels of solid-state image pickup devices and in the functionality of solid-state image pickup devices, there is a strong demand for enhanced functionality of transistors that are disposed in a peripheral circuit portion for controlling photoelectric conversion elements and reading out signals output from the photoelectric conversion elements. To enhance the functionality of the transistors, the widths of polysilicon electrodes that form gate electrodes and spaces therebetween have to be narrowed. The resulting narrow spaces between the electrodes have small (concave-shaped) hollows.
A method for forming the light-shielding gate electrodes described in Japanese Patent Laid-Open No. 2007-115803 includes patterning a polysilicon film by etching, thereby forming the gate electrodes. In the case where a polysilicon film is patterned, the polysilicon film may undesirably remain in narrow spaces between the gate electrodes of transistors arranged in the peripheral circuit portion. This remaining polysilicon film causes charge leakage and reduces yields. In order to improve the yields, the following method may be used. An insulating film disposed under the polysilicon film is thickened so as to make small concave-shaped hollows more gradual, thereby preventing the polysilicon film from remaining after etching. However, this method degrades the optical properties of the solid-state image pickup device because the distance between each photoelectric conversion element and a corresponding microlens disposed on the topmost surface increases. Such a drawback also occurs in another configuration described in Japanese Patent Laid-Open No. 2007-115803 in which the light-shielding film is formed of a non-polysilicon film, e.g., a metal film of tungsten, etc.
In addition, the same drawback may occur when a light-shielding film having a slit is formed in a solid-state image pickup device disclosed in Japanese Patent Laid-Open No. 2009-105358.